1. Field
Embodiments described herein relate generally to a semiconductor storage device such as NAND-type flash memory.
2. Background Art
Recent semiconductor memories tend to have smaller sizes and higher capacities. In particular, NAND-type flash memories are incorporated in many devices such as SD cards and solid state drives (SSDs). In such a circumstance, reliability degradation due to shrink are becoming manifest. A possible major cause of the degradation in the course of repeated writings and erasures is an electrical field stress imposed on tunnel oxide films of memory cells.
With the NAND-type flash memory, a Fowler-Nordheim (FN) tunnel electrical field is applied between the substrate and a charge accumulation layer of a memory cell to program or to erase charges into the charge storage layer. The memory cell stores data by associating the states of charging of the charge storage layer (a threshold voltage) with data “0” and “1”, for example.
In the course of the repeated writing and erase operations, the electrical field gradually damages the tunnel oxide film of the memory cell to increase defects therein. Thus, the charges leak from the charge storage layer through the defects in the tunnel oxide film, and the data retention capability degrades.